While there is a large body of literature suggesting that early word learning is critical for the later development of oral and written language, the processes by which problems in word learning interact with deficits in other aspects of linguistic knowledge are not well understood, nor is it clear whether deficits in vocabulary size impact later language development equally depending on their source. A better understanding of these interactions are critical because small vocabularies in early childhood place children at significant risk for delays in acquiring both spoken and written language. This study proposes to begin to fill in some of these gaps in our understanding by examining the relationships among components of phonological knowledge (including the ability to perceive speech sounds in challenging listening tasks, the ability to robustly differentiate among speech sounds in production, and the ability to make inferences about how sounds function in the language being acquired) and lexical acquisition in children with a wide range of vocabulary sizes, where size varies for different reasons across groups. The proposed research is a longitudinal study from 30 to 60 months of age of approximately 200 children with a wide variety of initial vocabulary sizes resulting from a range of advantages or deficits in the types of phonological knowledge that support word learning. The specific research question addressed is: what are the developmental relationships among vocabulary growth and three types of phonological knowledge (speech production knowledge, speech perception knowledge, and higher-level categorical knowledge) for children with normal hearing from middle-SES and low-SES families, and for children with cochlear implants (CIs)? The study will include three groups of children: one, children from low-SES families, who have smaller vocabularies than peers from middle-SES families because of more limited linguistic input (among other factors); two, children with cochlear implants, who have smaller vocabularies than peers with normal hearing because of their perceptual limitations; and three, children from middle-SES families with a wide range of vocabulary sizes, including a sizeable number of late talkers (i.e., children who have small expressive vocabularies at age 2, in spite of normal sensory functioning, normal cognitive development, and age-appropriate receptive vocabularies). The study also includes a statistical modeling component to determine optimal interventions for children with small-sized vocabularies. Structural equation modeling, based on data from this rich longitudinal sample, will allow us to test what interventions might be most effective to increase vocabulary for these different groups of children. Ultimately, the knowledge obtained from this study will enable us to develop efficacious and targeted early interventions for young children with atypically small vocabularies.